(1) Field of the Invention
The present invention generally relates to a hydraulic brake control apparatus, and more particularly to a hydraulic brake control apparatus which appropriately controls a brake fluid pressure in a brake system of an automotive vehicle.
(2) Description of the Related Art
A hydraulic brake control apparatus having two pressure supplying devices connected to wheel cylinders of a brake system in an automotive vehicle is known. For example, Japanese Laid-Open Patent Application No.1-278874 discloses this type of apparatus.
In the conventional apparatus of the above-mentioned publication, a master cylinder supplies a brake fluid pressure responsive to a brake operating force, and a pressure supplying device supplies a brake fluid pressure responsive to a drive signal. The pressure supplying device is controlled by the drive signal such that the brake fluid pressure supplied by the pressure supplying device is set at a brake fluid pressure equivalent to the brake operating force multiplied by a predetermined constant.
The conventional apparatus of the above-mentioned publication includes a pressure selection device. The pressure selection device selects a higher one of the pressure supplied by the master cylinder and the pressure supplied by the pressure supplying device when a brake pedal is depressed by the brake operating force by a vehicle operator. The higher pressure selected by the pressure selection device is supplied to the wheel cylinders.
The conventional apparatus of the above-mentioned publication includes a pressure reduction device. The pressure reduction device reduces the brake fluid pressure from the pressure supplying device when the pressure from the pressure supplying device is excessively high. The pressure reduction device supplies a reduced pressure to the pressure selection device for the selection of the brake fluid pressure to be supplied to the wheel cylinders.
According to the conventional apparatus of the above-mentioned publication, it is possible to smoothly increase the brake fluid pressure in the wheel cylinders by selectively using one of the master cylinder and the pressure supplying device after the brake pedal is depressed by the brake operating force. In addition, when the pressure supplied by the pressure supplying device is excessively high due to a defect in the pressure supplying device (this defect will be called the high-pressure defect), the conventional apparatus can prevent the supply of the excessively high pressure by the defective pressure supplying device directly to the wheel cylinders by using the pressure reduction device. Therefore, the conventional apparatus enables the brake system to quickly produce a braking force responsive to the brake operating force by the vehicle operator. Also, the conventional apparatus provides a fail-safe function when the high-pressure defect in the pressure supplying device has occurred.
However, the conventional apparatus of the above-mentioned publication requires the pressure reduction device in order to provide the fail-safe function against the high-pressure defect in the pressure supplying device. The production of the pressure reduction device needs a high cost, and it is difficult for the conventional apparatus of the above-mentioned publication to provide the fail-safe function with a low cost.
If the supply of the brake fluid pressure to the wheel cylinders by the pressure supplying device is inhibited and the supply of the brake fluid pressure to the wheel cylinders by the master cylinder is allowed when the high-pressure defect in the pressure supplying device has occurred, it is possible to control the brake fluid pressure in the wheel cylinders to be equal to the brake fluid pressure from the master cylinder. By taking this method, it is possible to prevent the brake fluid pressure in the wheel cylinders from being increased to the excessively high pressure due to the high-pressure defect.
A flow control device which carries out the above-mentioned method can be manufactured with a cost lower than the cost of the pressure reduction device. By using the flow control device, the fail-safe function against the high-pressure defect may be provided with a low cost.
However, when the high-pressure defect in the pressure supplying device has occurred, the brake fluid pressure in the wheel cylinders at that time is higher than the brake fluid pressure in the master cylinder. If the supply of the brake fluid pressure by the pressure supplying device is simply changed to the supply of the brake fluid pressure by the master cylinder when the high-pressure defect has occurred, the high-pressure brake fluid from the wheel cylinders may be returned back to the master cylinder. It is difficult to avoid the counter flow of the brake fluid into the master cylinder even when the above method is utilized, and the counter flow of the brake fluid into the master cylinder is detrimental to the durability of the master cylinder.
Accordingly, the above method which changes the supply of the brake fluid pressure by the pressure supplying device to the supply of the brake fluid pressure by the master cylinder when the high-pressure defect in the pressure supplying device has occurred, is not necessarily appropriate to provide the fail-safe function against the high-pressure defect. It is difficult for the above method to assure the durability of the master cylinder and provide the fail-safe function against the high-pressure defect.